Nozzle arrangement in airborne web-drying and method for improving heat transfer in airborne web-drying

ABSTRACT

A nozzle arrangement in an airborne web-drying apparatus for drying a coated paper web ( 10 ) or the like. The nozzle arrangement comprises at least one overpressure nozzle ( 14 ), which is arranged to blow drying air both in the web&#39;s travel direction and against the web&#39;s travel direction. The nozzle arrangement comprises further a direct impingement nozzle ( 16 ) combined with the exit side and/or the entrance side ( 26 ) of the overpressure nozzle, in which direct impingement nozzle a plurality of nozzle slots or nozzle orifices ( 17 ) are formed in order to blow drying air mainly perpendicularly toward the web. The perpendicular distance (a 1 ) from the nozzle surface ( 30 ) of the direct impingement nozzle ( 16 ) to the web is larger than the perpendicular distance (a 2 ) from the supporting surface ( 32 ) of the overpressure nozzle ( 14 ) to the web.

The object of the present invention is a nozzle arrangement in anairborne web-drying apparatus and a method for improving the heattransfer in airborne web-drying, the apparatus and the method beingdefined in the preambles of the independent claims presented below.

Then the object of the invention is typically a nozzle arrangement whichcomprises at least one overpressure nozzle extending transversely of theweb and having on both sides of the nozzle, i.e. on the entrance andexit sides of the nozzle, a nozzle slot extending across the web, inwhich case the nozzle slots on the opposite sides of the nozzle compriseone nozzle slot extending across the web or a row of successive nozzleorifices. The nozzle slots are arranged to blow drying air jetsobliquely against each other, or they are arranged to blow drying airjets, which are guided against each other with the aid of curvedCoanda-surfaces. The arrangement further comprises at least one directimpingement nozzle extending across the web, in which case a pluralityof nozzle slots or nozzle orifices are formed in this direct impingementnozzle for blowing drying air mainly perpendicularly against the web.Advantageously the nozzle orifices or slots of the direct impingementnozzle are arranged in one or more rows, or otherwise evenly distributedon the supporting surface of the direct impingement nozzle.

A plurality of overpressure nozzles or direct impingement nozzles aretypically arranged in an alternating succession on both sides of theweb. Thereby an overpressure nozzle and a direct impingement nozzle arearranged opposite each other, as shown e.g. in the international patentpublication WO 95/14199. In the solution presented in the WO-publicationthe space between each overpressure nozzle and the adjacent directimpingement nozzle forms a discharge passage for the wet discharge air.The discharge passages are ineffective regions regarding the drying ofthe web.

The aim is to continuously improve the effect of the airborne web-dryingfor instance in order to be able to make the drying faster and/or toreduce the size of the dryer. One economical means to improve the effectof airborne web-drying is to increase the nozzle temperature. However,it is not possible to increase the nozzle temperature in someapplications, or the desired effect can not be obtained with this singlemeasure.

The object of the present invention is to provide an improved nozzlearrangement and a method which are able to increase the effect ofairborne web-drying.

A particular object is to provide a nozzle arrangement which is easy torealise in airborne web-drying apparatuses of different types.

A further object is to provide an improved nozzle arrangement and methodwhich do not require substantial extra space for the airborne web-dryingapparatus.

In order to reach the above-mentioned objects the nozzle arrangement andmethod according to the invention in airborne web-drying arecharacterised in what is defined in the characterising parts of theindependent claims presented below.

The solution according to the invention uses nozzle assemblies which inthe same structure combine at least one overpressure nozzle and at leastone direct impingement nozzle. The assembly of overpressure nozzle anddirect impingement nozzle is advantageously mounted in a common framestructure and in a common nozzle box. The nozzle assembly comprisestypically an overpressure nozzle and a direct impingement nozzlearranged on both sides of the overpressure nozzle, i.e. on its entranceand exit sides. Thus no conventional discharge passage for wet air isformed between the overpressure nozzle and the direct impingementnozzles in the nozzle assembly. Compared to conventional solutions alarger part of the area of the dryer can in this way be utilised in theactual drying process. The discharge passages for the wet air arearranged between the different nozzle assemblies. Each passagedischarges drying air blown by both the overpressure nozzle and thedirect impingement nozzle. The direct impingement nozzles are arrangedin relation to the web, so that they do not hinder air from beingdischarged from the overpressure nozzle. The web will further facilitatethe air discharge from the direct impingement nozzle region in thetravel direction of the web.

In another typical solution according to the invention a directimpingement nozzle is arranged on the entrance or exit side in thetravel direction of the web of the over pressure nozzle and directlyattached to the overpressure nozzle, so that an assembly comprising anoverpressure nozzle and one direct impingement nozzle is formed.

The distance between the nozzle slots of the overpressure nozzle and thefirst nozzle orifice row closest to the overpressure nozzle isadvantageously >30 mm but <100 mm, typically 40 to 60 mm.

In conventional airborne web-drying solutions there is a relatively widedischarge air passage between each successive nozzle pair. Then theactual nozzles cover only less than half of the total area. In this casethere will be a poor heat transfer in the region of the discharge airpassage, as no air jets are directed at the web in this region. In thesolution according to the invention the drying utilises also a part ofthe empty space left between the individual nozzles in conventionaldryers. The direct impingement arranged in connection with theoverpressure nozzle enables an increased total amount of drying air tobe directed at the web, i.e. in this region the heat-transfercoefficient can be increased and the heat transfer can be made moreefficient. In measurements it was found that a considerably increasedheat transfer can be achieved with the solution according to theinvention. The heat transfer can be made more efficient with thesolution according to the invention, also when the temperature of thedrying air must kept very low, such as for instance in the drying of“thermal coatings”.

Each nozzle assembly according to the invention has typically nozzleorifices in one or two direct impingement nozzle sections, the nozzleorifices occupy an area having a total length of 20 to 250 mm in thetravel direction of the web, typically >50 mm, most typically >100 mm,or covering 10 to 60% of the length of the nozzle distribution. A directimpingement nozzle can of course also have only one row of nozzles ornozzle orifices, in which case the area is very small.

The nozzle orifices of the direct impingement nozzle parts havetypically a diameter of 2 to 10 mm, most typically about 5 mm, and thenozzles are arranged at a distance from each other which is 10 to 50 mm,typically 20 to 30 mm, both in the web cross direction and in the webtravel direction. The nozzle orifices are typically arranged in rows inthe cross direction of the web. There are typically 2 to 7 successiverows of nozzle orifices in the travel direction of the web.Advantageously the nozzle orifices in different rows are overlapping, sothat the total coverage of the orifices is as large as possible. Thenozzle orifices can also be arranged evenly on the supporting surface ofthe nozzle in other ways. An airborne web-drying apparatus containstypically several successive nozzle assemblies on both sides of the webto be dried.

In steam-heated dryers the heat source forms an upper limit for thetemperature. Also in this case the drying can be made more effectivewith the solution according to the invention. An effective nozzle canincrease the drying effect also in gas-heated dryers.

On the other hand the solution according to the invention can also beused in small spaces, particularly in short spaces, in order to maximisethe drying effect.

The gap between two successive assemblies according to the inventionforms a discharge passage for wet discharge air. The nozzle assembliesare disposed on different sides of the web to be dried, advantageouslyin such a manner that there is always a part of a nozzle assembly,preferably an overpressure nozzle part, on the other side of the webopposite to a discharge passage. The intention is to avoid a situationwhere two discharge passages would be located opposite each other. Theaim is that the web is guided at all points by drying air blows, atleast from one side of the web. An aim is also usually to arrange theoverpressure nozzles in the airborne web-drying apparatus so that theycause the web to travel forward like a sine wave.

In an advantageous nozzle arrangement solution according to theinvention the nozzle surface of the direct impingement nozzle, i.e. thesupporting surface of the nozzle, is at a longer perpendicular distancefrom the web line than the overpressure nozzle. The web line meanstypically a straight line located centrally between the drying boxes onopposite sides of the web. The web itself travels along the web line,but however, often like a sine wave. The distance of the nozzle surfaceof a direct impingement nozzle from the web line is advantageously 5 to40 mm, typically 10 to 15 mm, longer than the distance of the supportingsurface of an overpressure nozzle from the web line. The perpendiculardistance of the nozzle surface of a direct impingement nozzle from theweb line is typically about 20 to 30 mm. This ensures a discharge gasspace on the entrance and exit sides of the nozzle between the directimpingement nozzle and the web, for air blown from the nozzle slots onthe entrance and exit sides of the overpressure nozzle.

When the nozzle surface of the direct impingement nozzle is located at agreater distance from the web line than the nozzle surface or thesupporting surface of the overpressure nozzle, it is guaranteed that theair jets from the direct impingement nozzle part do not interfere withthe operation of the overpressure nozzle. Preferably the structure ofthe direct impingement nozzle and its air jets must be dimensioned, sothat the air jets turn suitably away from the overpressure nozzletoward, the discharge passage of the return air, i.e. the discharge air,and do not tend to form an obstruction to the air flow leaving theoverpressure nozzle.

The discharge passage between two adjacent nozzle assemblies isadvantageously dimensioned so that it can remove, regarding the traveldirection of the web

the discharge air from the exit side of the overpressure nozzle on theupstream side of the discharge passage, and the discharge air from thedirect impingement nozzle arranged on the exit side of this overpressurenozzle, and

the discharge air from the entrance side of the overpressure nozzle onthe downstream side of the discharge passage, and the discharge air fromthe direct impingement nozzle arranged on the entrance side of thisoverpressure nozzle.

The area of the discharge passage in the web direction is advantageouslyless than 40% of the corresponding total area of the airborne web-dryingapparatus, i.e. of the corresponding area covered by the nozzles and thedischarge passage.

The total area (A₁) of the openings of the direct impingement nozzle ornozzles in each direct impingement nozzle and overpressure nozzleassembly is typically

about 40 to 100% of the total area (A₂) of the nozzle slots of theoverpressure nozzle when there is a direct impingement nozzle only onone side, and

about 40 to 150% of the total area (A₂) of the nozzle slots of theoverpressure nozzle when there is a direct impingement nozzle on bothsides of the overpressure nozzle.

The width of the nozzle slots of the overpressure nozzles is typicallyabout 1.5 mm. The open area of the slots of the overpressure nozzles is1 to 2%, typically 0.8 to 1.5%, most typically about 1.2% of the totalarea of the airborne web-drying apparatus. The open area of the orificesof the direct impingement nozzles is correspondingly about 0.5 to 1.5%of the total area of the airborne web-drying apparatus. Sometimessmaller or larger opening areas can come into question.

In some cases, particularly when the width of the direct impingementnozzle in the web travel direction is relatively large, the nozzlesurface of the direct impingement nozzle arranged on the exit side ofthe overpressure nozzle can be curved, so that its distance from the webincreases in the travel direction of the web.

With the method according to the invention the heat transfer in airborneweb-drying can be effectively increased by blowing drying air directlyon the exit and/or entrance side of the overpressure nozzle, mainlyperpendicularly against the web, with the aid of a direct impingementnozzle having the nozzle surface at a larger distance from the web thanthe nozzle surface of the overpressure nozzle. Thus the solutionaccording to the invention ensures that the drying air blown from thenozzle slots on the exit side and/or the entrance side of theoverpressure nozzle and the drying air blown from the direct impingementnozzle form wet discharge air, which can be guided away from the webregion via a discharge passage formed on the exit side and/or entranceside of the direct impingement nozzle, without interfering with theoperation of the overpressure nozzle.

The invention is described in more detail below with reference to theenclosed drawings, in which

FIG. 1 shows, as seen from one side, an airborne web-drying apparatusprovided with a nozzle arrangement according to the invention;

FIG. 2 shows schematically a vertical cross-section in the web's traveldirection of one of the nozzle assemblies shown in FIG. 1;

FIG. 3 shows a cross-section according to FIG. 2 of another nozzleassembly;

FIG. 4 shows a cross-section according to FIG. 2 of a third nozzleassembly;

FIG. 5 shows schematically, as seen from one side and partly cut in theweb's travel direction, a part of an airborne web-drying apparatusprovided with a nozzle arrangement according to the invention;

FIG. 6 shows schematically a nozzle assembly according to the inventionin a cross-section along the web's travel direction and seen from above;

FIG. 7 shows according to FIG. 6 another nozzle assembly according tothe invention;

FIG. 8 shows according to FIG. 6 a third nozzle assembly according tothe invention; and

FIG. 9 shows according to FIG. 6 a fourth nozzle assembly according tothe invention.

FIG. 1 shows an airborne web-drying apparatus provided with anadvantageous nozzle arrangement according to the invention. In theairborne web-drying apparatus nozzle assemblies 12 are arranged bothabove and below the web 10, each nozzle assembly being formed by anoverpressure nozzle 14 and direct impingement nozzles 16, 16′ arrangedsymmetrically on both sides of the overpressure nozzle. A dischargepassage 18, 18′ for the discharge air is arranged in the gaps betweenadjacent nozzle assemblies.

In the case of FIG. 1 each overpressure nozzle 14 has two nozzle slots20, 22. A first or entrance side nozzle slot 20 is on the entrance side24 of the overpressure nozzle 14, and an exit side nozzle slot 22 is onthe exit side 26 of the nozzle. An entrance side direct impingementnozzle 16 is connected to the entrance side of the overpressure nozzle14, the direct impingement nozzle having nozzle orifices 17, and an exitside direct impingement nozzle 16′ is connected to the exit side, thisdirect impingement nozzle having nozzle orifices 17′. The air dischargefrom the nozzle slots 20, 22 and the nozzle orifices 17, 17′ isdescribed in more detail in connection with FIG. 5.

In each nozzle assembly 12 the air flowing from nozzle slots 20 on theentrance side of the overpressure nozzle and from the nozzle orifices 17of the direct impingement nozzle on the entrance side of thisoverpressure nozzle is discharged mainly through the discharge passage18 on the entrance side of the nozzle assembly. Correspondingly, in eachnozzle assembly 12 the air flowing from the nozzle slots 22 on the exitside of the overpressure nozzle, and from the nozzle orifices 17′ of thedirect impingement nozzle on the exit side of this direct impingementnozzle, is mainly discharged through the discharge passage (18′) on theexit side of the nozzle assembly.

With the direct impingement nozzles in this advantageous solution of theinvention the heat transfer can be intensified on both sides of theoverpressure nozzle. In addition the arrangement (geometry) of thenozzle assemblies according to FIG. 1 has proved very advantageousregarding the runnability in airborne web-drying. Different factorsaffect the good runnability. Firstly, in this arrangement the web issupported at all points by the blows, at least on one side of the web. Aweb which partly has to travel without any support will easily flutteras it finds its correct path of travel, which causes troubles regardingthe runnability. Secondly, in the solution according to FIG. 1 there isan overpressure nozzle on the opposite side of the web at each dischargepassage for wet air, i.e. at that point where suction is directed at theweb. This combined effect of suction and blow which is directed at theweb and guides the web, alternately upward and alternately downward,will cause a stable sine-wave shaped motion in the web. Thirdly, directimpingement nozzles are arranged on both sides of the overpressurenozzle, in which case the planar surfaces of the direct impingementnozzles on their part stabilise the travel of the web.

FIG. 2 shows in an enlarged cross-section the nozzle assembly 12according to FIG. 1, where a direct impingement nozzle part 16, 16′ isarranged on both sides of the overpressure nozzle part 14. As can beseen in FIG. 2 the nozzle assembly is an integrated structure. Thenozzle assembly has a common nozzle box 11.

Partitions 13 separating the entrance air side from the overpressurenozzle 14 are arranged in the nozzle box 11. That part 13′ of thepartition 13 which is directed toward the web forms the supportingsurface of the overpressure nozzle, which in the case of FIG. 2 isshaped as a Coanda surface. Inlet channels 14 a, 14 b are formed betweenthe partition 13 and both side walls of the nozzle box. The partitionhas openings 13″ at the inlet channels, and air flows from theseopenings into the overpressure nozzles.

The inlet channels 16 a and 16 b of the direct impingement nozzle partsare connected to both sides of the nozzle box 11. At these inletchannels 16 a, 16 b the nozzle box 11 has in its side walls openings 15,from which entrance air flows into the direct impingement nozzles. Thedirect impingement nozzle according to FIG. 2 has a planar nozzlesurface with nozzle orifices 17 in two adjacent rows.

The nozzle assembly according to FIG. 2 can be manufactured as a singlebeam-like structure, which is completely ready for installation andwhich makes the installation easier compared to conventional solutions,where each nozzle is brought as a separate part to the installation.Further it can be clearly seen in the figure that the nozzle assemblyhas a simple structure and that its manufacture and installationrequires substantially less material and fastening members than themanufacture and installation of three separate nozzles.

In a manner like that of FIG. 2 the FIG. 3 shows a nozzle assembly wherea direct impingement nozzle 16′ is connected only to one side of theoverpressure nozzle part 14, typically on the exit side. Theoverpressure nozzle structure is the same as in FIG. 2. The directimpingement nozzle structure is almost the same as in FIG. 2. However,in the solution of FIG. 3 the direct impingement nozzle part 16′ islarger than the corresponding nozzle part 16′ in the solution of FIG. 2.Further the nozzle part 16′ in FIG. 3 has three rows of nozzle orifices17 instead of two, in order to obtain a larger open area.

FIG. 4 shows in a similar way as FIG. 2 a third nozzle assembly 12. InFIG. 4 the nozzle box 11 has mainly a width equal to that of the nozzleassembly 12. In that part of the nozzle box which is toward the web thepartition 13 provided with openings forms two suction boxes, one box 16a for the nozzle orifices on the entrance side and another box 16 b forthe nozzle orifices on the exit side. From the air box 16 a on theentrance side the air flows both to the nozzle orifices of the directimpingement nozzle on the entrance side and to the nozzle slot of theoverpressure nozzle on the entrance side. Correspondingly, the air flowsfrom the air box 16 b on the exit side to the nozzle orifices of theoverpressure nozzle on the exit side and to the nozzle slot of theoverpressure nozzle on the exit side. Like the solution of FIGS. 2 and 3the partition forms the Coanda-surface of the overpressure nozzle.

FIG. 5, which for applicable parts uses the same reference numerals asFIG. 1, shows in more detail the paths of the air flows between thenozzle assembly and the web. In the case of FIG. 5 the air flows areillustrated as an example between the web and a nozzle assembly likethat of FIG. 3.

In an airborne web-drying apparatus using a nozzle assembly according toFIG. 2 the air flows will travel between the exit side of the nozzleassembly and the web mainly in the same way as in FIG. 5. The air flowsbetween the nozzle assembly according to FIG. 2 on the entrance side andthe web are mainly mirror images of the air flows on the exit side.

In the case of FIG. 5 nozzle assemblies 12 according to the inventionare arranged opposite each other on both sides of the web, so that anentrance side of a nozzle assembly and an exit side of a nozzle assemblyare located opposite each other on the opposite sides of the web. Inthis case the discharge passage 18 for wet air and the center of anozzle assembly will be located opposite each other on the oppositesides of the web.

In FIG. 5 on the entrance side 24 of the overpressure nozzle 14 there isa first slot or an entrance nozzle slot 20 and on the exit side 26 thereis an exit nozzle slot 22. From the entrance nozzle slot air isdischarged into the travel direction of the web, at a small angle αregarding the web. From the exit side nozzle slot air is dischargedagainst the travel direction of the web, at a small angle β regardingthe web. The air flows discharged from the overpressure nozzle riseabove the nozzle's supporting surface upwards toward the web, and turnthen into a direction which is mainly opposite to their dischargedirection, as shown by the thin arrows. The main part of the drying airdischarged from the nozzle orifice 22 on the exit side 26 is dischargedas wet discharge air or return air to the exit side of the nozzle 14 andfurther past the direct impingement nozzle through the discharge passage18 on the exit side. The main part of the drying air discharged from thenozzle orifice 20 on the entrance side 24 is discharged as wet dischargeair or return air through the discharge passage 18 formed on theentrance side of the nozzle. There may be a direct impingement nozzlepart between the overpressure nozzle 14 and the discharge passage 18.

From the direct impingement nozzle 16, connected to the exit side 26 ofthe over pressure nozzle, drying air flows through the nozzle orifices17 mainly perpendicularly against the web. The air turns in the webdirection and is discharged together with the air coming from theoverpressure nozzle as wet discharge air through the discharge passage18 arranged on the exit side 28 of the nozzle assembly 12, as shown bythe thin arrows.

The nozzle surface 30 of the direct impingement nozzle 16 is arranged sothat its distance a, from web is larger than the distance a₂ of thesupporting surface 32 of the overpressure nozzle 14 from the web.a₁−a₂=5 to 40 mm, typically 5 to 15 mm, advantageously about 10 mm.Supporting surface means that part of a nozzle which faces the web andwhich is limited to the region between the nozzle slots. Typically thesupporting surface is parallel to the web line direction. The surface ofthe nozzle can contain a recess below the supporting surface. The largerdistance between the direct impingement nozzle's nozzle surface orsupporting surface and the web enables the drying air from the exit sideof the overpressure nozzle to be discharged in the web's traveldirection. The nozzle surface (30) and the supporting surface (32) canalso be located at the same distance from the web, when desired.

FIG. 6 shows a nozzle assembly according to the invention, both in across section and in a top view. This figure uses the same referencenumerals as FIG. 1, when applicable. The distance between the nozzlesurface 30 of the direct impingement nozzle 16 and the web is a₁, andthe distance between the supporting surface 32 of the overpressurenozzle 14 and the web is a₂. The difference between these distancesa₁−a₂ is about 5 to 15 mm, advantageously about 10 mm.

The nozzle orifices 17 of the direct impingement nozzle 16 in FIG. 6 arearranged in three rows of nozzle orifices. FIG. 7 presents anothernozzle assembly according to the invention which differs from the formerone in that the direct impingement nozzle 16 has five nozzle rows. FIG.8 shows a third nozzle assembly according to the invention which differsfrom the former ones in that the direct impingement nozzle 16 has sevennozzle rows. The distance between the nozzle rows is about 20 to 30 mm.The distance between the nozzle orifices in the cross direction of theweb is about 20 to 30 mm.

In the direct impingement nozzle 16 of FIG. 8 a possible modification30′ of the nozzle surface 30 is drawn with broken lines. The nozzlesurface 30′ is arranged obliquely, so that its distance from the webincreases in the web's travel direction.

FIG. 9 shows a nozzle assembly which is similar to that of FIGS. 6 to 8,but which differs from the former in that a direct impingement nozzle16, 16′ is connected to both sides of the overpressure nozzle, in whichcase each direct impingement nozzle has two rows of nozzle orifices.However, the nozzle orifices can be located in only one row, or in morethan two rows. By using a nozzle assembly of this kind it is possible toincrease the heat-transfer coefficient both on the entrance side and onthe exit side of the overpressure nozzle.

The solution provides a more efficient heat transfer with the samevolume of drying air per square metre, which is considered to be animportant advantage of the invention. On the other hand, compared toconventional drying using overpressure nozzles, substantially higherheat transfer effects can be achieved with the same blowing velocity butusing a larger air volume per square metre, which is considered to beanother important advantage of the invention.

Tests have shown that a nozzle assembly according to the invention canincrease the heat-transfer coefficient on the section between the directimpingement nozzle and the web by about 100 W/m²/° C., compared to asituation which uses overpressure nozzles arranged one after another ina conventional manner, which leaves a discharge passage with a poor heattransfer between the nozzles. It has been found in the tests that thedirect impingement nozzles have no detrimental effects on the heattransfer at the overpressure nozzle.

An assembly of overpressure nozzles and direct impingement nozzles inthe same frame structure in the manner according to the invention willfurther provide substantial advantages in material saving, as well asadvantages regarding production techniques, installation techniques andthe amount of work.

With a suitable nozzle arrangement it is further possible to achieve ahighly stable web run and a good runnability, by arranging e.g. anoverpressure nozzle opposite the discharge passage for wet air, and bycombining a suitable direct impingement nozzle on the entrance side andthe exit side of the overpressure nozzle.

The invention is not intended to be limited to the above presentedembodiments, but the intention is to apply the invention widely withinthe inventive idea defined by the claims presented below.

What is claimed is:
 1. A nozzle arrangement in an airborne web-dryingapparatus for drying a coated fibre web, wherein the web has a traveldirection which defines an upstream direction and a downstreamdirection, comprising: a plurality of first nozzle assemblies positionedon a first side of the web and extending across the web, the firstnozzle assemblies defining first discharge passages therebetween whichextend across the web, the first discharge passages for wet dischargeair; a plurality of second nozzle assemblies positioned on a second sideopposite the first side of the web and extending across the web, thesecond nozzle assemblies defining second discharge passages therebetweenfor wet discharge air, the first nozzle assemblies and the second nozzleassemblies positioned so that each first discharge passage is opposite asecond nozzle assembly, and each second discharge passage is opposite afirst nozzle assembly; wherein each first nozzle assembly and eachsecond nozzle assembly comprises: at least one overpressure nozzleextending across the web, the at least one overpressure nozzle having afirst upstream nozzle slot extending across the web and a seconddownstream nozzle slot extending across the web, arranged to blow dryingair jets obliquely against each other; and at least one directimpingement nozzle extending across the web and having a plurality ofnozzle slots or nozzle orifices for blowing drying air mainlyperpendicularly against the web.
 2. The nozzle arrangement of claim 1wherein the first upstream nozzle slot and the second downstream nozzleslot of each nozzle assembly are guided against each other with the aidof curved Coanda-surfaces.
 3. The nozzle arrangement of claim 1 whereinthe at least one overpressure nozzle first upstream nozzle slot is asingle slot and the second downstream nozzle slot is a single slot. 4.The nozzle arrangement of claim 1 wherein each first upstream nozzleslot and second downstream nozzle slot comprises a row of successivenozzle orifices extending across the web.
 5. The nozzle arrangement ofclaim 1 wherein each first nozzle assembly and each second nozzleassembly comprises two direct impingement nozzles combined with the atleast one overpressure nozzle, one of said two direct impingementnozzles positioned on an upstream side of the at least one overpressurenozzle and a second of said two direct impingement nozzles positioned ona downstream side of the at least one overpressure nozzle.
 6. The nozzlearrangement of claim 5 wherein in each nozzle assembly the orifices ofsaid two direct impingement nozzles define a total area which is about40 to 150% of a total area defined by the first upstream nozzle slot andthe second downstream nozzle slot of the overpressure nozzle.
 7. Thenozzle arrangement of claim 1 wherein the first and second dischargepassages have a total area which is less than 40% of a total areadefined by the first nozzle assemblies, the second nozzle assemblies andthe first discharge passages and second discharge passages.
 8. Thenozzle arrangement of claim 1 wherein the at least one directimpingement nozzle of one of the nozzle assemblies has a nozzle surface,and defines a first perpendicular distance from the nozzle surface tothe web, and wherein said at least one of the nozzle assemblies hasportions of the overpressure nozzle which define a supporting surface,and wherein a second perpendicular distance is defined from thesupporting surface to the web, the first perpendicular distance being 5to 40 mm greater than the second perpendicular distance.
 9. The nozzlearrangement of claim 1 wherein the at least one direct impingementnozzle of each first nozzle assembly and each second nozzle assembly hasa nozzle surface, and wherein a first perpendicular distance is definedmeasured from the nozzle surface to a line defined by the web, and thefirst perpendicular distance is from about 20 mm to 30 mm.
 10. Thenozzle arrangement of claim 1 wherein in each of said first nozzleassemblies and said second nozzle assemblies, the distance between thesecond downstream nozzle slot of the each overpressure nozzle and theclosest downstream nozzle slot or nozzle orifices of the at least onedirect impingement nozzle is greater than 30 mm, and less than 100 mm.11. The nozzle arrangement of claim 1 wherein the at least one directimpingement nozzle has nozzle nozzle slots or nozzle orifice in aregion, defined in the travel direction of the web, which has a lengthof 20 to 250 mm.
 12. The nozzle arrangement of claim 1 wherein thenozzle orifices of the at least one direct impingement nozzle arearranged in two to seven rows which are successive in the traveldirection of the web, and wherein the nozzle orifices in successive rowsare arranged in an overlapping manner.
 13. The nozzle arrangement ofclaim 1 wherein the diameter of the nozzle orifices of the at least onedirect impingement nozzle is about 2 to 10 mm, and wherein the width ofthe first and second nozzle slots of the at least one overpressurenozzle is about 1.5 mm.
 14. The nozzle arrangement of claim 1 whereinthe orifices of the at least one direct impingement nozzle define atotal area which is about 40 to 100% of a total area defined by thefirst and second nozzle slots of the overpressure nozzle.
 15. The nozzlearrangement of claim 1 wherein in each nozzle assembly the firstupstream nozzle slot and the second downstream nozzle slot of theoverpressure nozzle have an area which is 1 to 2%, of an area defined bythe overpressure nozzle, and the nozzle slots or nozzle orifices of thedirect impingement nozzle define an open area of about 0.5 to 1.5%, ofan area defined by the direct impingement nozzle.
 16. The nozzlearrangement of claim 1 wherein each first nozzle assembly and eachsecond nozzle assembly is arranged so that one direct impingement nozzleis arranged on an upstream side one overpressure nozzle.
 17. The nozzlearrangement of claim 1 wherein each first nozzle assembly and eachsecond nozzle assembly is arranged so one direct impingement nozzle isarranged on a downstream side of one overpressure nozzle.
 18. The nozzlearrangement of claim 1 wherein the direct impingement nozzle defines asurface in which are formed the orifices of the direct impingementnozzle, the surface being inclined, so that its distance from the webincreases in the travel direction of the web.
 19. A method for fordrying a coated fibre web comprising the steps of: passing a coatedfibre web, passing from upstream to downstream between a plurality offirst nozzle assemblies positioned on a first side of the web andextending across the web, the first nozzle assemblies defining firstdischarge passages therebetween which extend across the web, the firstdischarge passages for wet discharge air, and a plurality of secondnozzle assemblies positioned on a second side opposite the first side ofthe web and extending across the web, the second nozzle assembliesdefining second discharge passages therebetween for wet discharge air,the first nozzle assemblies and the second nozzle assemblies positionedso that each first discharge passage is opposite a second nozzleassembly, and each second discharge passage is opposite a first nozzleassembly; wherein each first nozzle assembly and each second nozzleassembly comprises: at least one overpressure nozzle extending acrossthe web and having an upstream side and a downstream side, theoverpressure nozzle having a first upstream nozzle slot extending acrossthe web and a second downstream nozzle slot, which blow drying air jetsobliquely against each other; and at least one direct impingement nozzleextending across the web, having a plurality of nozzle slots or nozzleorifices which blow drying air mainly perpendicularly against the web;and discharging wet air formed by the drying air jets of theoverpressure nozzle and the drying air of the direct impingement nozzleaway from the web through the first and second discharge passages. 20.The method of claim 17, wherein each direct impingement nozzle has anozzle surface positioned a first distance from the web, and whereineach overpressure nozzle has a surface defined between the firstupstream nozzle slot and the second downstream nozzle slot, and thedistance between each overpressure nozzle surface and the web is lessthan the first distance.
 21. The method of claim 17, wherein at leastone direct impingement nozzle is arranged on a downstream side of eachoverpressure nozzle, and wherein wet air, formed by the drying air blownfrom the downstream nozzle slot of the overpressure nozzle and thedrying air from the direct impingement nozzle arranged on the downstreamside of the overpressure nozzle, is guided away from the web through oneof said first and second discharge passages which is formed downstreamof the direct impingement nozzle.
 22. The method of claim 17, whereinwet air, formed by the drying air blown from the upstream nozzle slot ofeach overpressure nozzle, is guided away from the web through one ofsaid first and second discharge passages which is formed upstream of thedirect impingement nozzle.
 23. A nozzle arrangement in an airborneweb-drying apparatus for drying a coated fibre web, wherein the web hasa travel direction which defines an upstream direction and a downstreamdirection, comprising: a plurality of first nozzle assemblies positionedon a first side of the web and extending across the web, the firstnozzle assemblies defining first discharge passages therebetween whichextend across the web, the first discharge passages for wet dischargeair; a plurality of second nozzle assemblies positioned on a second sideopposite the first side of the web and extending across the web, thesecond nozzle assemblies defining second discharge passages therebetweenfor wet discharge air, the first nozzle assemblies and the second nozzleassemblies positioned so that each first discharge passage is opposite asecond nozzle assembly, and each second discharge passage is opposite afirst nozzle assembly; wherein each first nozzle assembly and eachsecond nozzle assembly comprises: at least one overpressure nozzleextending across the web, the at least one, overpressure nozzle having afirst upstream a means for blowing air extending across the web and asecond downstream means for blowing air extending across the web,arranged to blow drying air jets obliquely against each other; and atleast one direct impingement nozzle extending across the web and havingmeans for blowing drying air mainly perpendicularly against the web.